1. Field of the Invention
The present invention relates to optical data storage systems. In particular, the present invention is an objective lens velocity control system for use in an optical data storage system.
2. Description of the Prior Art
Optical data recording technology has developed to the point where it is commonly found in many consumer electronic products. Optical video disks and optical compact audio disks have, for example, become very popular. This technology is also being adapted to high density optical data recording and storage systems. With continued advances in this technology, it is believed that optical systems of this type will be capable of surpassing magnetic data storage systems currently in widespread use in terms of both performance and cost.
Optical data recording systems of the types referred to above include a record carrier, or disk, on which arcuate servo tracks are positioned. A laser beam which is focused by an objective lens is used to write data onto, and to read data from, the servo tracks. A tracking drive system is used to drive the objective lens along a tracking axis, thereby moving the objective lens and laser beam to desired servo tracks on the record carrier.
Included within the tracking drive system is a velocity control system for controlling the velocity of the objective lens along the tracking axis as it is driven between servo tracks. Velocity control systems of the prior art typically implement a sequential three-phase velocity control cycle including measurement, calculation, and correction-output phases.
During the measurement phase, a tracking error signal representative of the position of the objective lens relative to individual servo tracks along the tracking axis is monitored to determine the actual velocity of the objective lens. The velocity control system then calculates the difference between the actual velocity and a desired velocity during the calculation phase. During the correction-output phase, the velocity control system produces a tracking drive signal as a function of the difference between the actual and desired velocities, and applies this tracking drive signal to a tracking actuator which drives the objective lens along the tracking axis.
The prior art velocity control system discussed above has inherent phase delays which reduce overall stability of the system and introduce errors. Even though the calculation and correction-output phases of a cycle can be performed during the measurement phase of the following cycle, there are always delays between the measurement phase and the correction-output phase due to the finite amount of time required to perform the calculation and correction-output phases. Velocity of the objective lens has likely changed between an end of a measurement phase and the beginning of a correction-output phase. As a result, the tracking drive signal is not accurately correcting for the current velocity of the lens. This phase delay results in a loss of stability and control over the objective lens. Although one technique to reduce this phase delay is to implement the velocity control system with a faster microprocessor, this adds to the overall system cost and increases its power consumption.
Clearly, there is a continuing need for improved velocity control systems for use in optical data storage systems. A velocity control system which corrects for errors in the velocity of the objective lens during measurement phases would be especially desirable. Phase delays associated with the calculation and correction-output phases can thereby be eliminated. Stability and control of the velocity control system would thereby be enhanced. Of course, the velocity control system must also be relatively inexpensive and cost effective.